What a Speech Disorder Reveals About Brain Function

The beginning of a new year means the beginning of a new term at University of California, San Diego and another request to deliver another guest lecture in an Introduction to Cognitive Science course. While I complain about having to do this every quarter, I actually enjoy giving the lecture. It gives me an opportunity to talk about one of the most compelling phenomena in cognitive neuroscience: aphasia.

Aphasia is a communication disorder that results from brain damage, typically to the left side of the brain. Besides being inherently fascinating, aphasia is important in the history of neuroscience because it enabled scientists to infer a relationship between a complex behavior, the ability to speak, and the normal function of a particular region of the brain, thefrontal region of the left hemisphere.

Paul Broca, a 19th century French neurologist, istypically credited with discovering aphasia during his treatment of a mysterious patient known only as “Monsieur Leborgne” or by his nickname, Tan. Although Leborgne (whose full name and identity were revealed in 2013) showed signs of understanding what was said to him, and was able to control his mouth and tongue, he was unable to produce fluent speech. For example, if asked to open his mouth or stick out his tongue, he could do so, but if asked a question he typically replied with two or more utterances of the syllable “tan” (hence the nickname).

After Leborgne died, Broca inspected his brain and found a hole in the front portion of his left hemisphere. Remarkably, a few months later, Broca encountered another patient, Monsieur Lelong, whose speech was confined to five words (the French words for “yes”, “no”, “three” and “always”, as well as “Lelo,” a mispronunciation of his own name). Monsieur Lelong also died soon after Broca examined him, and Broca found a lesion—an area of abnormal tissue—in the front portion of Monsieur Lelong’s left hemisphere. This area of the brain, part of the left inferior frontal gyrus, is now known as Broca’s area, and the communicative disorder that Broca reported has come to be known as Broca’s aphasia.

Broca’s contribution to cognitive neuroscience was his argument that complex abilities such as the ability to speak could be localized to particular regions of the brain. Because the ability to speak is supported by the left inferior frontal gyrus, damage to this part of the brain causes a speech disorder. The patients Broca treated had a very severe form of aphasia, in that their speech was limited to only a few words. The modern definition of Broca’s aphasia, however, includes patients with a range of symptoms. Patients with Broca’s aphasia, also known as expressive aphasia, have trouble speaking fluently, pronouncing words correctly and finding the right words to express themselves. They also have difficulty if asked to repeat words that are said to them.

The most common cause of aphasia is a stroke and the brain damage that results from bleeding in the brain or a blood clot that prevents part of the brain from receiving oxygen. In the following clip, a bright young woman named Sarah Scott describes a stroke she suffered as a teenager.

In general, her speech is easy to understand, though she does have difficulty articulating the word “leg”. Her utterances are quite short, though, and often don’t form complete sentences. This is perhaps the main characteristic of Broca’s aphasia: short, telegraphic utterances. When I show the clip below of another Broca’s aphasic in class, students often ask whether aphasics are aware that their speech is not normal. As you’ve probably already guessed from the clip of Sarah Scott, most Broca’s aphasics are painfully aware of their difficulty putting their thoughts into words.

When my students see video clips of aphasic patients struggling to find words, they often think about strategies the patients could use to get around their problems. For example, students often ask whether patients with Broca’s aphasia can write down what they are trying to say. Sometimes aphasic patients can write down a single word that they are unable to say, so having a pen and paper ready can help them communicate with others. In the clip above, Sarah Scott uses this technique. However, patients with Broca’s aphasia tend to write and talk in a similar fashion. If they are unable to speak in complete sentences, they are unable to write in complete sentences, and just as their speech consists of nouns (“arm”, “leg”) with very few verbs (“hurt”), so does their writing.

Another suggestion is that patients with Broca’s aphasia could learn sign language to compensate for their problems speaking. However, signed languages, like spoken ones, have a vocabulary of tens of thousands of signs, and involve rules for grammatically combining them into sentences. Unless the patient already knew sign language, it’s very unlikely that someone with a severe communicative deficit could learn a whole new language. Individual signs (or spontaneously invented gestures) could be used to help counter word-finding deficits experienced by Broca’s aphasics. Some patients do use gestures, either to help themselves think of elusive words, or to allow their listeners to guess what they mean. But the same factors that make it difficult for speakers to string words together into sentences apparently make it difficult for them to combine a series of signs into sentences. Deaf signers who suffer from strokes affecting the left frontal lobe often develop Broca’s aphasia, signing slowly and hesitantly in short telegraphic utterances.

Another common question students have is whether these patients can get better. The answer is that it depends on how severe their stroke was and how old they are. In general, the smaller the lesion, the greater a patient’s chance of recovering some speech function. Also, younger people are more likely to recover from a stroke than older people because the brain’s plasticity, or ability to “rewire” itself, is greatest during childhood and declines with age. Interestingly, most Broca’s aphasics do gradually get better in the weeks, months and years that follow their strokes. Compare the following clip of Sarah Scott to the first one.Read more at kavliblog.org